The invention relates to antennas, and more particularly, to a stagger tuned meanderline loaded antenna.
Efficient antennas typically require structures with minimum dimensions on the order of a quarter wavelength of their intended radiating frequency. Such dimensions allow an antenna to be easily excited and to be operated at or near its resonance, limiting the energy dissipated in resistive losses and maximizing the transmitted energy. These conventional antennas tend to be large in size at their resonant wavelengths. Moreover, as the operating frequency decreases, antenna dimensions tend to increase proportionally.
To address shortcomings of traditional antenna design and functionality, the meanderline loaded antenna (MLA) was developed. A detailed description of MLA techniques is presented in U.S. Pat. No. 5,790,080. Wideband MLAs are further described in U.S. Pat. Nos. 6,323,814 and 6,373,440, while narrowband MLAs are described in U.S. Pat. No. 6,373,446. An MLA configured as a tunable patch antenna is described in U.S. Pat. No. 6,404,391. Each of these patents is herein incorporated by reference in its entirety.
Generally, an MLA (also known as a xe2x80x9cvariable impedance transmission linexe2x80x9d or VITL) is made up of a number of vertical and horizontal conductors. The vertical and horizontal sections are separated by gaps at certain locations. Meanderlines are connected between at least one of the vertical and horizontal conductors at the corresponding gaps. A meanderline is made up of alternating high and low impedance sections, and is designed to adjust the electrical (i.e., resonant) length of the antenna.
In addition, the design of the meanderlines provide a slow wave structure that permits lengths to be switched into or out of the circuit. Such switching changes the effective electrical length of the antenna with negligible electrical loss. The switching is possible because the active switching devices are located in the high impedance sections of the meanderline. This keeps the current through the switching section low, resulting in very low dissipation losses and high antenna efficiency.
A conventional meanderline loaded antenna generally provides a symmetrical coverage pattern (e.g., figure eight). Horizontal polarization, loop mode, is obtained when the antenna is operated at a frequency that is a multiple of the full wavelength frequency, which includes the electrical length of the entire line, comprising the meanderlines. Such an antenna can also be operated in a vertically polarized, monopole mode, by adjusting the electrical length to an odd multiple of a half wavelength at the operating a frequency. The meanderlines can be tuned using electrical or mechanical switches to change the mode of operation at a given frequency or to switch the frequency when operating in a given mode.
A general limitation on performance of antennas and radiating structures is governed by the Chu-Harrington relation for small lossy, conducting spheres: Efficiency=64VQ where: Q=Quality Factor V=Volume of the structure in cubic wavelengths. Thus, antennas achieve an efficiency limit of the Chu-Harrington relation as their dimensions diminish. However, given the proliferation of applications using wireless technology, there is an on-going need for smaller and more efficient antennas.
What is needed, therefore, are techniques for improving antenna efficiency or otherwise extending the Chu-Harrington relation.
One embodiment of the present invention provides a meanderline loaded antenna configured for stagger tuning. The antenna includes a first vertical radiator adapted with a feed point and having first and second ends. The first end is operatively coupled to a reference plane. The antenna further includes a second vertical radiator having first and second ends, with the first end operatively coupled to the reference plane at a distance from the first vertical radiator. A horizontal radiator having first and second edges is also included. The horizontal radiator is located in relation to the first and second vertical radiators so as to define a gap between each edge of the horizontal radiator and the second end of each vertical radiator. A pair of meanderlines is also included, with each interconnecting one of the vertical radiators to the horizontal radiator across the corresponding gap. The meanderlines are adapted for causing a combination of loop mode and monopole mode current distribution thereby enabling antenna quality factor adjustment substantially independent of antenna gain.
Another embodiment of the present invention provides a method for tuning a meanderline loaded antenna. The antenna is configured with a pair of vertical radiators spaced at a distance from each other, and a horizontal radiator is located in relation to the vertical radiators so as to define two gaps. A meanderline is connected between the horizontal radiator and the corresponding vertical radiator across each gap. The method includes decreasing delay associated with one of the meanderlines as compared to delay associated with the other meanderline thereby causing a combination of loop mode and monopole mode current distribution, and enabling antenna quality factor adjustment substantially independent of antenna gain. The method further includes monitoring antenna performance to determine if a desired gain and quality factor are achieved.
Another embodiment of the present invention provides a method of manufacturing a meanderline loaded antenna configured for stagger tuning. The method includes providing a pair of vertical radiators spaced at a distance from each other, with each vertical radiator having an upper edge. The method further includes providing a horizontal radiator having first and second edges, the horizontal radiator located in relation to the vertical radiators so as to define a gap between each edge of the horizontal radiator and the upper edge of each vertical radiator. The method also includes providing a pair of meanderlines, each meanderline interconnecting one of the vertical radiators to the horizontal radiator across the corresponding gap. Each meanderline is adapted to stagger tune the antenna thereby enabling antenna quality factor adjustment substantially independent of antenna gain.
The features and advantages described herein are not all-inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and not to limit the scope of the inventive subject matter.